Experimental study of water-based drilling fluid disturbance on natural gas hydrate-bearing sediments

Natural gas hydrates (NGH) are widely considered as a promising important substitute energy resource. Some countries are trying to drill and develop NGH; however, the disturbance effect of water-based drilling fluid on the stability of NGH-bearing sediments is unclear, especially the water-based drilling fluid with a thermodynamics hydrate inhibitor. To understand the disturbance effect, this paper studies the affecting patterns of drilling fluid temperatures and thermodynamics hydrate inhibitors on NGH-bearing samples with different hydrate saturations through an experimental study of drilling disturbance induced by water-based drilling fluids and by measuring the acoustic velocity variation of NGH-bearing sediments. The results show that the acoustic velocity of the natural gas hydrate-bearing sediments increases with the growing hydrate saturation. The gas hydrate dissociation is accelerated by the increase in drilling fluid temperatures. However, this acceleration effect declines after hydrate saturation exceeds 50%. The NGH dissociation inside the formation is also affected by the micro distribution of natural gas hydrates. Addition of thermodynamics hydrate inhibitors into the drilling fluid can apparently boost the gas hydrate dissociation, and the greater the concentration of inhibitors, the more obvious the effect. The boost effects of salt-type inhibitors are markedly stronger than the effects of alcohol-type inhibitors. This means that the better inhibition effect of the inhibitors is, the greater the disturbance effect on NGH-bearing sediments. Therefore, the drilling fluid that is suitable for NGH-bearing sediments should not only be capable of suppressing the NGH generation in the wellbore but also should have eased stimulation of the NGH dissociation inside the drilled formation. The findings will provide help with drilling fluid design that is suitable for NGH-bearing sediments and provide a scientific basis for achieving safe drilling through NGH-bearing sediments.